Switching the nature of catalytic centers in Pd/NHC

A well?established oxidative addition of organic halides (R?X) to N?heterocyclic carbene (NHC) complexes of palladium(0) leads to formation of (NHC)(R)Pd
II(X)L species, the key intermediates in a large variety of synthetically useful cross?coupling reactions. Typical consideration of the cross?coupling catalytic cycle is based on the assumption of intrinsic stability of these species, where the subsequent steps involve coordination of the second reacting partner. Thus, high stability of the intermediate (NHC)(R)PdII(X)L species is usually taken for granted. In the present study it is discussed that such intermediates are prone to non?classical R?NHC intramolecular coupling process (R = Me, Ph, Vinyl, Ethynyl) that results in removal of NHC ligand and generation of another type of Pd catalytic system. DFT calculations (BP86, TPSS, PBE1PBE, B3LYP, M06, wB97X?D) clearly show that outcome of R?NHC coupling process is not only determined by chemical nature of the organic substituent R, but also strongly depends on the type of solvent. The reaction is most favorable in polar solvents, whereas the non?polar solvents render the products less stable.